fix: support commodity/forex/crypto tickers and never invent prices (#781)

Analyzing a symbol Yahoo Finance does not recognize (e.g. XAUUSD+) could
produce an invented price instead of an error. The agent now either prices
the correct instrument or clearly reports that data is unavailable.

Ticker support:
- Commodities/forex/crypto resolve to the symbol Yahoo actually serves, so
  you can enter the common form and it just works:
    XAUUSD / XAUUSD+ / GOLD  -> GC=F   (gold)
    USOIL                    -> CL=F   (WTI crude)
    EURUSD                   -> EURUSD=X
    BTCUSD                   -> BTC-USD
    SPX500 / NAS100          -> ^GSPC / ^NDX
  Native Yahoo symbols (AAPL, GC=F, ^GSPC) keep working unchanged. New
  instruments are added by extending the alias table.

Reliability:
- Unknown or delisted symbols now return a clear "data unavailable" result
  the agent reports verbatim, instead of a value the model fills in.
- A failed fetch no longer leaves a broken symbol cached until the cache is
  cleared by hand.

tests/test_symbol_utils.py

@@ -0,0 +1,81 @@
+"""Tests for symbol normalization and the no-data routing sentinel."""
+
+import unittest
+
+import pytest
+
+from tradingagents.dataflows.symbol_utils import (
+    NoMarketDataError,
+    normalize_symbol,
+    is_yahoo_safe,
+)
+
+
+@pytest.mark.unit
+class TestNormalizeSymbol(unittest.TestCase):
+    def test_plain_equities_unchanged(self):
+        for sym in ("AAPL", "MSFT", "TSM", "BRK.B", "0700.HK", "^GSPC", "GC=F"):
+            self.assertEqual(normalize_symbol(sym), sym)
+
+    def test_lowercases_are_upper(self):
+        self.assertEqual(normalize_symbol("aapl"), "AAPL")
+        self.assertEqual(normalize_symbol("  msft  "), "MSFT")
+
+    def test_metal_aliases_map_to_futures(self):
+        self.assertEqual(normalize_symbol("XAUUSD"), "GC=F")
+        self.assertEqual(normalize_symbol("XAUUSD+"), "GC=F")   # broker CFD suffix
+        self.assertEqual(normalize_symbol("xauusd+"), "GC=F")
+        self.assertEqual(normalize_symbol("GOLD"), "GC=F")
+        self.assertEqual(normalize_symbol("XAGUSD"), "SI=F")
+
+    def test_energy_and_index_aliases(self):
+        self.assertEqual(normalize_symbol("USOIL"), "CL=F")
+        self.assertEqual(normalize_symbol("SPX500"), "^GSPC")
+        self.assertEqual(normalize_symbol("NAS100"), "^NDX")
+        self.assertEqual(normalize_symbol("US30"), "^DJI")
+
+    def test_forex_pairs_get_x_suffix(self):
+        self.assertEqual(normalize_symbol("EURUSD"), "EURUSD=X")
+        self.assertEqual(normalize_symbol("GBPJPY"), "GBPJPY=X")
+        self.assertEqual(normalize_symbol("eurusd"), "EURUSD=X")
+
+    def test_crypto_pairs_get_dash_usd(self):
+        self.assertEqual(normalize_symbol("BTCUSD"), "BTC-USD")
+        self.assertEqual(normalize_symbol("ETHUSD"), "ETH-USD")
+
+    def test_six_letter_non_currency_left_alone(self):
+        # GOOGLE-style 6-letter tickers that aren't two currency codes
+        # must not be mangled into a fake forex pair.
+        self.assertEqual(normalize_symbol("ABCDEF"), "ABCDEF")
+
+    def test_empty_input_passthrough(self):
+        self.assertEqual(normalize_symbol(""), "")
+
+
+@pytest.mark.unit
+class TestNoMarketDataError(unittest.TestCase):
+    def test_message_includes_resolution(self):
+        err = NoMarketDataError("XAUUSD+", "GC=F", "no rows")
+        self.assertIn("XAUUSD+", str(err))
+        self.assertIn("GC=F", str(err))
+        self.assertEqual(err.symbol, "XAUUSD+")
+        self.assertEqual(err.canonical, "GC=F")
+
+    def test_canonical_defaults_to_symbol(self):
+        err = NoMarketDataError("FOOBAR")
+        self.assertEqual(err.canonical, "FOOBAR")
+
+
+@pytest.mark.unit
+class TestIsYahooSafe(unittest.TestCase):
+    def test_accepts_structural_chars(self):
+        for sym in ("AAPL", "GC=F", "^GSPC", "BRK.B", "BTC-USD"):
+            self.assertTrue(is_yahoo_safe(sym))
+
+    def test_rejects_slash_and_space(self):
+        for sym in ("a/b", "AA PL", ""):
+            self.assertFalse(is_yahoo_safe(sym))
+
+
+if __name__ == "__main__":
+    unittest.main()